Youngest is 17, oldest is 31. But it was the younger ones when they were around 10 - I think they were just mathematically calculating the middle third. I’m almost in their “old” category now and think that because (fit) people are aging more slowly than past generations middle age is stretching out, if you are defining it as able bodied and working. That stretches it to like 75 for some people. I don’t think over 30 is “young” though, so if there are only 3 categories it’s middle aged, and no way is 75 not old, if you are fit, healthy, and working at that age you are a fit old person.
And who can’t rock a bikini at 30? WTF, where do you live?